Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., Arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as Angina Pectoralis and Incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and Epilepsy. Further, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
Each of these implantable neurostimulation systems typically includes at least one stimulation lead implanted at the desired stimulation site and an Implantable Pulse Generator (IPG) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via one or more lead extensions. Thus, electrical pulses can be delivered from the neurostimulator to the electrodes carried by the stimulation lead(s) to stimulate or activate a volume of tissue in accordance with a set of stimulation parameters and provide the desired efficacious therapy to the patient.
The neurostimulation system may further comprise a handheld Remote Control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. The RC may, itself, be programmed by a technician attending the patient, for example, by using a Clinician's Programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon.
IPGs are routinely implanted in patients who are in need of Magnetic Resonance Imaging (MRI). Thus, when designing implantable neurostimulation systems, consideration must be given to the possibility that the patient in which neurostimulator is implanted may be subjected to electro-magnetic forces generated by MRI scanners, which may potentially cause damage to the neurostimulator as well as discomfort to the patient.
In particular, in MRI, spatial encoding relies on successively applying magnetic field gradients. The magnetic field strength is a function of position and time with the application of gradient fields throughout the imaging process. Gradient fields typically switch gradient coils (or magnets) ON and OFF thousands of times in the acquisition of a single image in the presence of a large static magnetic field. Present-day MRI scanners can have maximum gradient strengths of 100 mT/m and much faster switching times (slew rates) of 150 mT/m/ms, which is comparable to stimulation therapy frequencies. Typical MRI scanners create gradient fields in the range of 100 Hz to 30 KHz, and Radio Frequency (RF) fields of 64 MHz for a 1.5 Tesla scanner and 128 MHz for a 3 Tesla scanner.
In an MRI environment, the radiated RF fields may impinge on an IPG and cause different types of problems. For example, MRI-induced heating via the stimulation lead(s) is a risk incurred by patients implanted with IPGs. While the mechanism and degree of heating is a complicated issue, it has clearly been demonstrated in tissue phantoms that electrodes with faulty connections or compromised insulation may tend to exacerbate the heating problem. This problem is made more challenging if the faulty connections are intermittent, such that an electrode integrity check performed prior to the MRI may not indicate a defective electrode that may otherwise be temporarily compromised during the MRI (say due to a slight postural shift while the patient is undergoing the MRI). Therefore, even though IPGs may be designed to be MRI-compatible or the MRI procedure is well-controlled to limit the heat generation, patients may still be exposed to risk if the stimulation lead integrity breaks down. In addition, MRI safety studies for neurostimulation systems, such as cardiac pacemakers, have demonstrated that the electrode-tissue coupling efficiency can change from a pre-MRI procedure to a post-MRI procedure.
There, thus, remains a need to minimize tissue damage or discomfort of the patient due to heating of faulty stimulation leads during an MRI procedure, as well as ensuring that effective stimulation is provided to the patient during or after an MRI procedure.